Wednesday, 27 August 2008

The OCUnit framework is bundled with a standard xcode installation, which is handy for getting up and running with unit tests and test-driven development (TDD). Somewhat annoyingly though it does not play nicely with the Cocoa Touch framework. So, how does one go about building iPhone applications using TDD? I'm sure there are several ways of achieving this, but I wanted to stick with OCUnit. After a bit of messing about this is how I do it.

The Cocoa Touch framework encourages the use of the model-view-controller (MVC) pattern. It's only the 'V' and 'C' bits however that need to extend Cocoa Touch objects (UIView and UIViewController); you're completely free to implement the 'M' bit as you see fit. So, the basic approach I take is to write the model classes against OCUnit tests compiled against the (Cocoa) Foundation library only, and to then use these same classes in my iPhone application, which is compiled against the Cocoa Touch framework. The Foundation library exists in both the Cocoa and Cocoa Touch frameworks and is (almost) identical, although the unit tests only run against the Cocoa version.

This setup involves creating two projects: a Foundation Tool project and a Cocoa Touch project, and to reference the model code (which lives in the Foundation Tool project) directly from the Cocoa Touch project. Rather than duplicating the files that comprise the model, they are just referenced from the Cocoa Touch project (which is achieved by dragging them from the finder into the Cocoa Touch project, making sure the 'Copy items into destination group's folder' option is not checked).

The files that comprise the model objects should import Foundation/Foundation.h whereas the files in the Cocoa Touch project should generally import UIKit/UIKit.h (which is a higher-level header file that ultimately includes Foundation/Foundation.h).

What this means is that unit tests only cover the model, not the controllers or the views - so domain logic should be in the model, with the controllers simply acting as conduits for responses from this logic. In other words, there should be as little domain logic as possible in the controllers (and obviously the views, but there should not be any logic in the views anyway!)